1. Field of the Invention
The present invention relates to an apparatus for sintering a glass preform for an optical fiber and to a method thereof for preventing a leakage of gas within a muffle tube as well as an entry of external air into the muffle tube, and for extending the service life of the muffle tube so as to reduce cost.
2. Description of the Related Art
The processes referred to as a VAD (vapor-phase axial deposition) process and an OVD (outside vapor deposition) process are known as being processes for manufacturing a glass preform for an optical fiber. In these processes, first, a glass raw material is burnt in flames to generate glass particles and such particles are deposited on a rotating target rod either axially or radially, and then a porous glass preform is produced.
The porous glass preform produced in this way is heated to 1400-1600° C. in an atmosphere wherein the major component thereof is helium and wherein dehydration gas, such as chlorine for reducing OH groups in glass, and oxygen for reducing binding defects in glass, are added as needed. The reason why helium is used is because it has high thermal conductivity and also because it has a high solubility into glass and thus, bubbles are unlikely to remain in the glass. Prior to transparent vitrification, a dehydration process may be performed at the temperature of 1000-1250° C. The porous glass preform is heated to 1400-1600° C. and becomes a transparent glass preform.
The heating of the porous glass preform is carried out in a vessel made of quartz glass, which is referred to as a muffle tube, in an apparatus referred to as a sintering furnace. A heating furnace is arranged around the central part of the muffle tube. At the upper end portion of the muffle tube, a shaft is passed therethrough in order to suspend the porous glass preform. The porous glass preform is heated sequentially from its end portion by passing, in a rotating manner, the heating zone in an upward direction or a downward direction.
While the glass preform is heated at the central part of the muffle tube, convection tends to occur inside the muffle tube since space exists at the upper portion or lower portion of the glass preform, and thus, the internal pressure in the muffle tube fluctuates.
Since the shaft passes through the pass-through portion at the upper end of the muffle tube in a rotational and vertical manner, a complete gas-tight sealing is difficult, and when the internal pressure in the muffle tube becomes negative, the external air will be sucked into the muffle tube through the pass-through portion. When the external air is sucked into the muffle tube, the moisture in the external air will also be taken therein and thus, the concentration of OH groups in the glass will be increased, which leads to a degradation of the transmission properties of the optical fiber. In addition, nitrogen gas, which has a larger molecular mass than that of helium, will be taken therein and thus, bubbles tend to remain in the glass, which leads to a decrease in product yield.
In contrast, when the internal pressure in the muffle tube becomes positive, chlorine, which is a component of the atmospheric gas in the muffle tube, will be released into the external air. Since chlorine is a toxic and corrosive gas, it is unpreferable for work environments. Chlorine will further shorten the service life of the apparatus by oxidizing the same and the oxidization of the apparatus will lead to dust arising, which leads to a degradation of the product's properties and a decrease in product yield.
Moreover, when the pressure difference between outside and inside of the muffle tube becomes large, the muffle tube made of quartz glass will shrink or expand due to such pressure difference. When the muffle tube shrinks, the maximum external diameter of the porous glass preform processable in the muffle tube will be decreased. Further, when the muffle tube expands and sticks to the heating furnace, the service life of the muffle tube will be reduced.
In order to solve the problems above, in Patent Document 1, three chambers, respectively referred to as a pressure adjustment gas introduction part, an exhaust chamber and a sealing chamber, are provided at the upper lid portion; a flow rate of the supplied atmospheric gas is controlled in accordance with pressure fluctuations in the muffle tube; a flow rate of helium, which is supplied as pressure adjustment gas, is controlled in accordance with pressure fluctuations in the exhaust chamber; and nitrogen is supplied to the sealing chamber as sealing gas.